In studying oil recovery processes, cores are often used in order to predict the behaviour of the reservoir to a particular recovery technique. An important reservoir property to be duplicated in the core is the wettability or wetting preference of the reservoir solids. Changes in the wettability of the core have been shown to affect electrical properties, capillary pressure, waterflood behaviour, relative permeability, dispersion and simulated enhanced oil recovery. In order for the core analysis to predict the behaviour of the reservoir, the wettability of the core must be the same as the wettability of the undisturbed reservoir rook.
There are several methods, quantitative and qualitative, for measuring wettability of a reservoir core. Three quantitative measurements are most often used: (1) contact - angle measurement, (2) the Amott method (involving imbibition and forced displacement), and (3) the U.S. Bureau Mines method. These methods and others are described and compared in W.G. Anderson, "Wettability Literature Survey", J. of Petroleum Technology", 1246 (1986). Such wettability or wetting preference measurements typically provide an indication of whether the solid surface is preferentially oil-wet or preferentially water-wet.
Generally speaking, the above techniques are best suited to measuring the wettability of a consolidated surface or a consolidated core. No method, to the inventors' knowledge, exists for measuring the wettability of a dispersed medium, i.e. particulate solids dispersed in a multiphase liquid medium such as oil and water.
Electroacoustical methods and apparatus are known for measuring the electro-kinetical properties of liquids, colloidal suspensions or slurries, see for example U.S. Pat. No. 4,294,696 issued Oct. 13, 1981 to Beck et al.; U.S. Pat. No. 4,497,208 issued Feb. 5, 1985 to Oja et al.; U.S. Pat. No. 4,552,019 issued Nov. 12, 1985 to Freeman; and A.J. Babchin et al., "Elecotrokinetic Measurements by Electroacoustical Methods", Adv. Colloid Interface Sci., 30, 111 (1989). These references disclose techniques to measure electroacoustical signals proportional to electrokinetic potentials of dispersed particles etc. (i.e. signals Which correlate sonic and electrical fields). However, electrokinetic potentials are only very weakly related to wetting properties of solids. Therefore, no conclusions on wetting properties can be made on the basis of a direct measurement electrokinetic potential.